Conventionally, optical disks such as a CD, a DVD, and a BD (Blu-ray Disc) are widely used as optical recording media for recording various types of information including video and sound. With an optical information device using such an optical recording medium, since information is recorded or reproduced by irradiating the optical recording medium with light, information recording density is dependent on a size of an optical spot that converges on the optical recording medium. Therefore, capacity enlargement of an optical recording medium can be achieved by reducing an optical spot irradiated by an optical pickup. The size of the optical spot is proportional to a numerical aperture of an objective lens and inversely proportional to a wavelength of irradiated light. Thus, a smaller optical spot can be formed by either further shortening the wavelength of the light used or further increasing the numerical aperture of the objective lens.
However, with optical information devices already put into practical use, a distance between an optical recording medium and an objective lens is sufficiently greater than wavelength. In addition, when a numerical aperture of an objective lens exceeds 1, light incident to the objective lens is fully reflected at a lens emission plane. Therefore, recording density of optical recording media could not be increased.
In consideration thereof, a near-field optical recording/reproducing method using an SIL (a solid immersion lens) has been developed as an optical recording/reproducing method that is applicable in a case where an objective lens has a numerical aperture exceeding 1. If n denotes a refractive index of a medium of an optical recording medium and θ denotes a maximum angle of incident light with respect to an optical axis, then a numerical aperture NA can be defined by NA=n·sin θ. Normally, when the numerical aperture exceeds 1, an angle of light emitting the objective lens equals or exceeds a critical angle. Light in a region equal to or exceeding the critical angle is fully reflected at an emission end plane of the objective lens. The fully-reflected light seeps out from the emission end plane as an evanescent light. The near-field optical recording/reproducing method is configured such that the evanescent light propagates from the lens to the optical recording medium. Therefore, a spacing (an air gap) between the emission end plane of the objective lens and a surface of the optical recording medium is kept shorter than an attenuation distance of the evanescent light so that light in a range where a numerical aperture exceeds one is transmitted from the objective lens to the optical recording medium.
With an optical system using such a solid immersion lens, in order to propagate light in the form of evanescent light, a spacing between the solid immersion lens and an optical disk must be kept sufficiently shorter than a wavelength of light. For example, the spacing between the solid immersion lens and the optical disk must be set to approximately 1/10 of the wavelength of light or less, which means that when using light with a wavelength of 405 nm, the spacing between the solid immersion lens and the optical disk must be kept to around 25 nm. However, when there is a relative inclination between the solid immersion lens and the optical disk in such a narrowly-spaced state, an end of the solid immersion lens and the optical disk collide with each other. Therefore, a margin of error permissible for inclination is extremely small.
A relative inclination angle θ between the solid immersion lens and the optical disk is expressed by Expression (1) below. In Expression (1) below, g denotes a spacing between the solid immersion lens and the optical disk and D denotes a diameter of a tip of the solid immersion lens. When the diameter D of the tip of the solid immersion lens is set to 40 μm and the spacing g is set to 25 nm, the permissible relative inclination angle θ is around 0.07 degrees.θ=sin−1(g/2D)  (1)
However, limiting the relative inclination angle to or below 0.07 degrees is not easy. One method of limiting the relative inclination angle involves detecting the relative inclination angle and inclining the solid immersion lens or the optical disk. As a method of detecting a relative inclination angle, a method is proposed in which a distribution of a reflected light from an end face of the solid immersion lens is detected and the relative inclination angle is detected from a bias in the distribution of the reflected light (for example, refer to Patent Literature 1).
FIG. 47 is a diagram showing a configuration of a conventional optical pickup. A beam outputted from a semiconductor laser 401 is converted into a parallel light by a collimator lens 402 and is transmitted through a beam splitter 403 and a beam splitter 404. The beam having passed through a quarter wavelength plate 405 is converted into a convergent light by a lens 406a. The beam now in the form of a convergent light is incident to a solid immersion lens 406b and converges on an optical disk 407. A tip of the solid immersion lens 406b and a surface of the optical disk 407 are in proximity with each other at a distance where light is propagated in the form of evanescent light.
The beam reflected by the optical disk 407 once again passes through the solid immersion lens 406b, the lens 406a, and the quarter wavelength plate 405, and is incident to the beam splitter 404. A part of the beam incident to the beam splitter 404 is reflected and is incident to an optical detector 408. Another part of the beam incident to the beam splitter 404 is transmitted and is incident to the beam splitter 403. The beam incident to the beam splitter 403 is reflected toward an optical detector 409 and is incident to the optical detector 409. At this point, the optical detector 408 receives the beam reflected by an information face of the optical disk 407 and generates a signal for information reproduction. On the other hand, the optical detector 409 receives light reflected by an end face of the solid immersion lens 406b. In addition, the optical detector 409 has a four-fraction light receiving section. Each light receiving section outputs a signal corresponding to a quantity of respectively received light.
FIG. 48 is an enlarged view of a vicinity of the end face of the solid immersion lens 406b in a case where the end face of the solid immersion lens 406b and a surface of the optical disk 407 are inclined relative to each other in the conventional optical pickup. A peripheral light depicted by an arrow A and a peripheral light depicted by an arrow B differ from each other in distances between the end face of the solid immersion lens 406b and the surface of the optical disk 407. Therefore, a position where the peripheral light depicted by the arrow A passes and a position where the peripheral light depicted by the arrow B passes have different reflectances. Accordingly, light-dark differences occur in a beam reflected by the end face of the solid immersion lens 406b. The optical detector 409 shown in FIG. 47 is able to detect an inclination angle by detecting the light-dark differences as differences in signal quantities among the four light receiving sections.
In addition, as another method of detecting a relative inclination angle, a method is proposed in which a plurality of beams are irradiated on an optical disk through an end face of a solid immersion lens to detect a relative inclination angle (for example, refer to Patent Literature 2).
However, with the conventional configuration described above, when conceivably using only one light source on a multilayered disk having a plurality of recording layers to obtain a reproduction signal from the recording layers, a gap signal for gap control, and a tilt signal for detecting a relative inclination angle, a beam diameter at the end face of the solid immersion lens varies according to a position of the recording layer on which information is to be recorded or from which information is to be reproduced. As a result, a detection sensitivity of a relative inclination angle varies significantly. In particular, using a solid immersion lens is problematic in that the thinness of a cover layer of the optical disk prevents a practical tilt detection sensitivity from being obtained at a recording layer nearest to the surface.
In addition, when changing a layer on which a beams is focused (for example, a recording layer on which information is recorded or from which information is reproduced) in a multilayered disk, a conventional method of using a plurality of beams is problematic in that detection cannot be carried out by sufficiently separating a main beam and a sub-beam from each other.